Add small section of domain and challenge for signed VPs to README.

README.md

@@ -43,8 +43,10 @@ to "wrap" the implementation in a minimal VC-API implementation. Example code
 for which is available at https://github.com/Wind4Greg/Server-for-VCs
 Additionally your verifier will need to be able to verify Verifiable Credentials
 and Verifiable Presentations signed with `eddsa-rdfc-2022`. We also recommend
-that any issuer endpoints submitted to this test suite also issue using 
-`eddsa-rdfc-2022`.
+that any issuer endpoints submitted to this test suite also issue using
+`eddsa-rdfc-2022`. VerifiablePresentations will be submitted for verification
+both signed and unsigned. Signed VPs from this suite will have a domain and challenge set.
+The domain should be the test repo and the challenge is static.
 
 The issuer endpoint will need to conform to the
 [VC Issuer API](https://w3c-ccg.github.io/vc-api/#issue-credential).